The Solar System and light-years

The Earth is one of eight planets orbiting the Sun, which is just one star among billions.

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The Solar System

The Sun is a star at the centre of our Solar System. Eight planets orbit it. In order from the Sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
The four inner planets are small and rocky. The four outer planets are giants made mostly of gas and ice.
Moons orbit planets. Pluto was reclassified as a dwarf planet in 2006.
Gravity keeps the planets in orbit around the Sun and the Moon in orbit around the Earth.

Weight on other worlds

weight = mass × gravitational field strength (g)
The value of g depends on the planet or moon.
Your mass stays the same everywhere; your weight changes.
Placeg (N/kg), approximately
Earth10
Moon1.6
Mars3.7
Jupiter25

Light-years

Light travels at about 300 000 km per second.
Light from the Sun takes about 8 minutes to reach us.
A light-year is the distance light travels in one year: about 9.5 million million km. It is a unit of distance, not time.
The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away. Our galaxy, the Milky Way, is about 100 000 light-years across.
Worked example

An astronaut has a mass of 70 kg. What is her weight on Mars, where g is 3.7 N/kg?

  1. weight = mass × g
  2. 70 × 3.7

Answer: 259 N

Key idea

Eight planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Weight = mass × g, and g changes from place to place. A light-year is a distance: how far light travels in a year.

Key words
Weight
The force of gravity on a mass, measured in newtons (N). Weight = mass × gravitational field strength.
Mass
The amount of matter in an object, measured in kilograms (kg). It is the same everywhere.
Light-year
The distance light travels in one year. It is a unit of distance, not time.
Gravitational field strength
On Earth g = 10 N/kg; on the Moon about 1.6 N/kg.

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  1. Which planet is furthest from the Sun?

    1. Mercury
    2. Earth
    3. Jupiter
    4. Neptune
    Show the answer

    Answer: Neptune

    Neptune is the eighth planet.

  2. A probe has a mass of 50 kg. What is its weight on Jupiter, where g is 25 N/kg?

    Show the answer

    Answer: 1250 N

    weight = 50 × 25 = 1250 N

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.